The University of Alabama Birmingham (UAB) Center for Biophysical Sciences & Engineering (CBSE) develops low temperature freezers to facilitate the transportation and processing of experiments to the International Space Station (ISS) in accordance with NASA. The UAB CBSE began as “a bio protein crystal growth engineering unit,” says Daniel Sealy, Mechanical Engineer at CBSE. “We evolved into designing and building the storage and temperature control units for protein groves. Slowly, we developed into producing incubators and freezers capable of reaching -20ºC. The most current unit, Polar, is only -95°C, but our Glacier unit is capable of -160°C.” After funding for protein crystal research dropped off, CBSE continued through strong engineering successes with the Merlin, Glacier and Polar freezers. Recently, protein crystal research has returned to CBSE’s focus with planned experimentation on the ISS through Polar.

Polar’s preliminary purpose is transportation of samples to and from the station, with the ultimate goal long term residence aboard the ISS. There are currently three active Polar freezers with a total of fourteen full active units set for a future date. Space on the space station is limited and carefully regulated to optimize usage. The freezers, which will be kept in a single mid deck locker unit, have strict volume requirements to accommodate experiment samples on the inside and stringent volume restrictions to conform to exterior space. In order to maximize the usage of the volume allotted for the Polar freezer - and maximize the space for storing samples and adequately supporting them - the team of engineers and scientists from UAB CBSE turned to 3D printing.

"We had to go through a lot of learning about how to design for 3D printing and what types of geometry worked well, but it seems to be working great."

The main design challenge facing CBSE’s most recent unit, Polar, began with its interior liner. The team originally planned on manufacturing a liner similar to previous freezers, such as the liner used within the Glacier freezers: Flat machined pieces of PEEK joined together. Machining flat pieces and joining them caused the formation of sharp corners, which jammed inefficiently within Polar’s curved inner unit, resulting in an inefficient use of space within the limited interior space of Polar.

The second solution the team experimented with involved thermoforming a PEEK liner with curved corners to match the inside of the unit. This created further problems, mainly inaccurate tolerances and fluctuations in sheet thicknesses. Additionally, thermoforming necessitated the use of many brackets, joining pieces and mounting points for various electrical parts needed in the interior volume. It could be accomplished, but was a less than ideal use of space.

During one meeting, 3D printing was discussed as a potential solution to both the problems thermoforming and machining posed. At the time, the team was already using 3D printing for the ductwork on Polar. 3D printing with Fused Deposition Modeling (FDM) is commonly used in aerospace ducting, especially in conjunction with the high performance thermoplastic ULTEM. “We discussed whether 3D printing would be feasible for the inner shell,” explains Sealy.

“At first, we continued the line of thinking similar to thermoforming, in which many pieces would be printed and joined together. But after some research, we realized there were printers with a large enough printing volume that the entire shell could be printed as a single piece. This led us to where we are now. We had to go through a lot of learning about how to design for 3D printing and what types of geometry worked well, but it seems to be working great.”

Fused Deposition Modeling (FDM) 3D printing from additive manufacturing company Stratasys Direct Manufacturing was used to build the freezer’s interior shell using a printer with a large build platform of 36” x 24” x 36”. FDM creates parts by laying out layer after layer of heated thermoplastics via a precise nozzle according to computer instructions. Sealy and his team used ULTEM 9085 material in conjunction with FDM for a durable protective inner shell. ULTEM is a material best recognized for its resistance to heat and chemicals, with a heat deflection temperature around 153ºC (at 264psi).

Using ULTEM in an extreme cold environment is an uncommon application, but the material has passed testing and the team is confident in its performance. “As this isn’t a structurally critical area, we did minimal testing,” says Sealy. “But it does have to withstand the rocket vibrations as it journeys to the station.” Sealy and his team did 3 point bend tests on the ULTEM 9085 material, freezing it and simulating the stress it’d be expected to experience. “We dropped it from certain heights, bent it - generally made sure it wouldn’t shatter, chip or break,” explains Sealy.

“Once the first unit was built, it also had to undergo vibration testing on a shake table that simulated what it would see during launches to verify that the entire design didn’t have any issues, and the Ultem performed well with no issues in that testing.” The material, which will be subjected to continuous -80ºC, performed as hoped. 3D printing with ULTEM provided the team with a compact inner shell that contained all the mounting structures necessary without requiring tons of little parts, thereby providing the team with optimal usage of the volume allotted.

The Polar freezer will house an amalgam of scientific samples, largely biological - such as blood, urine and cells - on the space station itself to study effects of zero gravity on human cells over time among other experiments. The first freezer units reached the International Space Station in February aboard the Falcon 9 SpaceX CRS-5.
This article appeared on www.EngineeringCapacity.com

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